INTRODUCTION. 1 1 



Mr. Roozeboom's discussion of isomorphous mixtures seems 

 admirably verified by this investigation of the feldspars. When 

 considered in connection with the high viscosity of the materials, it 

 also explains the fact that the curve of melting points closely follows 

 Kuster's rule. It woidd seem, therefore, that not only concentrated 

 solutions but isomorphous ones form exceptions to the accepted laws 

 of dilute aqueous solutions. Such isomorphous solutions as those of 

 the feldspars here dealt with could in fact hardly be considered as 

 dilute, and some of them (as for instance AbiAiii) must be very 

 concentrated solutions, whichever of the components is considered as 

 solvent. 



The specific volumes of the feldspars seem to bear a relation to the 

 composition so nearly linear that the differences may be ascribed 

 to unavoidable errors in synthesis and analysis. It should not be 

 forgotten, however, that the specific volumes are determined at 

 something like iooo below the temperature of crystallization, and 

 that, since the coefficients of contraction of Ab and An doubtless 

 differ to some extent, variations in density as determined at 25 

 might be due not to a lack of isomorphism, but to the difference in 

 contraction of the two components. 



The artificial feldspars prepared at the cost of great labor are pure, 

 while natural crystals are not so. Hence lithologists, in making 

 separations by heavy solutions, should substitute the densities here 

 found for those hitherto employed. The changes are not great, but 

 they are sufficient in some cases to affect conclusions. 



A very noteworthy result of the investigation is the apparent super- 

 heating of the albitic feldspars. It is pointed out by Messrs. Day and 

 Allen that this may be only apparent and due to the extreme viscosity 

 of the melt. In fact, the separation of molecules in melting and their 

 deorientation must be successive processes, so that in any fusion, if 

 the operation could be instantaneously arrested, a layer of molecules 

 would be found separated from the solid mass but not yet deoriented. 

 Such material would differ from a liquid crystal by not being in a 

 condition of stable equilibrium. 



Prof. J. P. Iddings kindly undertook the detailed examination of 

 the slides made from the feldspar preparations. He shows in his 

 report that to one per cent, or less, the feldspars correspond optically 

 to the mixtures prepared. A closer correspondence could not be 

 hoped for in materials so viscous that diffusion afforded scarcely any 

 aid in attaining homogeneity. He has discussed many interesting 

 features of the crystallization of the feldspars, most of them familiar 



